1. Field of the Invention
The present invention relates to a focus detecting system for detecting focusing condition of an image forming optical system to an object, and, more particularly, a focus detecting system of this kind in which there are produced two signals corresponding to imaging conditions of object images at two positions substantially on opposite sides, front and rear, of and at substantially equal distances from a predetermined focal plane of the image forming optical system to be focused onto the object. These signals are compared to detect the focusing condition of said optical system to the object, particularly, in-focus or defocus condition, together with the defocusing direction in case of said defocus condition, namely near-focus or far-focus.
2. Description of the Prior Art
Heretofore, there have been proposed many methods and devices for detecting in-focus condition of an image forming optical system by evaluating sharpness of an image formed by said optical system. In one of the focus detecting systems heretofore proposed, a photo-sensitive element having output characteristics varying in accordance with the variation of image sharpness are arranged at two positions on substantially opposite sides of and at substantially equal distances from a predetermined focal plane of said image forming optical system, and at the time of adjustment of said optical system the outputs of these two photo-sensitive elements are compared with each other to decide that said optical system is under an in-focus condition when said outputs become equal to each other. According to such system, it is possible to decide the focusing condition, namely in-focus or defocus condition, as well as the defocusing direction in the latter case, namely near-focus or far-focus condition. Particularly, such system is advantageous in the optical system provided with a servo mechanism to perform automatic focusing operation. In practice, however, a good imaging condition cannot be always assured in all cases, and, on the contrary, bad imaging condition may be produced in all cases. In the latter case, it may be impossible to obtain a correct decision on the in-focus condition and an erroneous focusing result may occur.
Particularly, such system has a fundamental defect in that it is very difficult to distinguish correctly focused condition from considerably defocused condition. That is, this system is constructed to detect the focusing condition by comparing the values of outputs of the above-mentioned photo-sensitive elements, and the outputs of both photo-sensitive elements become equal to each other when the optical system is correctly focused onto the object, while these outputs of both photo-sensitive elements also become equal or substantially equal in the case where the optical system is under a considerably defocused condition.